1. Field of the Invention
The present invention relates to steam supply in a nuclear power plant. Steam moves from a nuclear reactor to a steam turbine through a main steam system with main steam isolation valves for isolating the nuclear reactor.
2. Description of the Related Art
Japanese Non-examined Patent Publication No. 9-80195 discloses a steam turbine control device for a nuclear power plant. FIG. 8 is a basic block diagram of the main steam system and a turbine by-pass system of a nuclear power plant.
Steam generated in a nuclear reactor 1 is supplied to a steam turbine 8 through a main steam system 61. There are a plurality of, for example ten, main steam isolation valves (MSIV) 2 settled inside and outside of a primary containment vessel (PCV) 3 containing a nuclear reactor 1, and a main steam header 4, a main steam stop valve 5, and a main steam control valve (CV) 6, in the main steam system 61.
The main steam isolation valves 2 operate independently from other pressure control valves to isolate the nuclear reactor 1. The main steam isolation valves 2 are open in the usual state of operation, and close to automatically seal the inside and outside of the primary containment vessel 3 when there is possibility that reactor coolant may flow out of the primary containment vessel 3. The main steam isolation valves 2 thus close, for example, if an accident internal or external of the primary containment vessel 3 arises.
The main steam stop valve 5 blocks the steam from reaching a steam turbine 8 when suspending the steam turbine 8. The main steam control valve 6 adjusts the discharge of steam that is generated in the nuclear reactor 1 and that flows into the steam turbine 8. The steam then rotates the steam turbine 8, and a generator 9, directly linked with the steam turbine 8, generates an electric output A turbine by-pass system 62, independent of the main steam system 61, branches from the main steam system 61 at the main steam header 4 and connects to a condenser 10 via the turbine by-pass valve 7.
A steam system supplies drive steam for a turbine gland steam evaporator 12, a reactor feed water pump turbine 13, and a steam jet air ejector 14, from the main steam system 61. The turbine gland steam evaporator 12 supplies a little steam to the space of a gland sealing part of the steam turbine 8, i.e., the boundary portion with the open air, in order to seal the gland part of a turbine 8. The steam jet air ejector 14 continuously extracts noncondensing gases, such as hydrogen and oxygen, from air in a condenser 10 or in exhaust gas of the steam turbine 8. The steam jet air ejector performs an action like spraying the steam supplied from the turbine by-pass system 62 and sends the noncondensing gas to an off-gas system and thereby maintains the vacuum of the condenser 10.
During normal operation, the main steam control valve 6 adjusts the reactor dome pressure, when a reactor dome pressure detector 11, installed in the nuclear reactor 1, detects the reactor dome pressure. A turbine by-pass valve 7 is kept completely closed then.
If an accident occurs at a startup or a shutdown of the nuclear power plant, or a electric transmission system, the position of the main steam control valve 6 is restricted, and the turbine by-pass valve 7 adjusts the pressure of the nuclear reactor 1.
FIG. 9 is a block diagram for explaining the conventional example of the steam turbine control device of the nuclear power plant of FIG. 8.
The steam turbine control device described below controls the positions of the main steam control valve 6 and the turbine by-pass valve 7.
A reactor dome pressure signal from the reactor dome pressure detector 11 is inputted into the steam turbine control device 19 and is compared with the pressure setting of the reactor dome pressure setter 23. A first pressure deviation calculating unit 24 then calculates the pressure deviation. A reactor dome pressure control calculating unit 25 receives the pressure deviation calculated by the first pressure deviation calculating unit 24 and sends a signal proportional to the deviation as a pressure control signal 29 to a first low value selector 18.
The first low value selector 18 compares the pressure control signal 29 to a speed/load control signal from a speed/load control calculating unit 15, a load restriction signal of a load limiter 16, and a maximum discharge restriction signal from a maximum discharge restriction unit 17. The speed/load control signal from the speed/load control calculating unit 15 controls the speed, i.e., the rotational number of the steam turbine 8, and the load of the generator 9, i.e., the electric output. The first low value selector 18 then chooses the lowest value signal among these signals and outputs it as a position demand of the main steam control valve 6.
Moreover, a first deviation calculating unit 20 generates the deviation signal between the pressure control signal 29 calculated by the reactor dome pressure control calculating unit 25 and the position demand signal to the main steam control valve 6. A second deviation calculating unit 21 generates the deviation signal between the maximum discharge restriction signal calculated by the maximum discharge restriction unit 17 and the position demand of the main steam control valve 6. The two deviation signals from the first and second deviation calculating units 20, 21 are inputted into the second low value selector 22, which outputs the lower value of the two deviation signals as a position demand signal of the turbine by-pass valve 7.
In addition, the reactor dome pressure detectors 11 are generally multiplexed to improve reliability, and in FIG. 9, a first medium value selector 27 selects a medium value of the triplex reactor dome pressure detectors 11 as a signal to be used for control.
In the conventional steam turbine control device of the nuclear power plant described above, in a usual operating state, the main steam control valve 6 is adjusted, based on the pressure signal from the reactor dome pressure detectors 11 installed in the nuclear reactor 1, to control and fix the pressure of the nuclear reactor 1. But in that case, if an accident detected by, for example, a reactor isolation signal detector (not shown) inside or outside of the primary containment vessel 3 occurs, and if the main steam isolation valves 2 are automatically in a fully closed position at the time of the accident, the pressure of the nuclear reactor 1, i.e., the reactor dome pressure, will rise abruptly.
In this case, the main steam control valve 6 and the turbine by-pass valve 7 open, and the drive steam of the turbine gland steam evaporator 12, the reactor feed water pump turbine 13, and the steam jet air ejector 14 decrease abruptly. FIGS. 10a-10c are signal time charts for explaining this situation.
If the main steam isolation valves 2 in FIG. 8 are fully closed, the pressure signal from the reactor dome pressure detectors 11 installed in the nuclear reactor 1 goes up as shown in FIG. 10a. In FIG. 10a, the ordinate axis shows pressure and the abscissa axis shows time.
Since at that time the pressure deviation which is the output of the first pressure deviation calculating unit 24 of the steam turbine control device 19 rises, the pressure control signal 29 calculated by the reactor dome pressure control calculating unit 25 goes up as shown in FIG. 10c In FIG. 10c, the ordinate axis shows an output of the signal and the abscissa axis shows time. The output of the first low value selector 18 goes up until it is restricted by either the speed/loadcontrol signal, the load restriction signal, or the maximum discharge restriction signal. Then, the main steam control valve 6 will open according to an increase of the pressure control signal 29.
On the other hand, if the first low value selector 18 restricts the pressure control signal 29, the position demand signal to the main steam control valve 6 also becomes restricted, and the deviation signal between the pressure control signal 29 and the position demand signal to the main steam control valve 6 calculated by the first deviation calculating unit 20 goes up.
Therefore, since the output of the second low value selector 22 goes up until it is restricted by the deviation signal between the pressure control signal 29 and the maximum discharge restriction signal calculated by the second deviation calculating unit 21, the position demand signal to the turbine by-pass valve 7 goes up as shown in FIG. 10c, and the turbine by-pass valve 7 will open.
FIG. 10b is a signal time chart of the reactor dome pressure control signal 36. In FIG. 10b, the ordinate axis shows the output of the signal and the abscissa axis shows time.
If the main steam control valve 6 and the turbine by-pass valve 7 open as mentioned above, since the steam remaining in the main steam system 61 downstream of the main stream isolation valves 2 flow into the steam turbine 8 or are directly collected by the condenser 10, the drive steam of the turbine gland steam evaporator 12, the reactor feed water pump turbine 13, and the steam jet air ejector 14, i.e., main steam pressure, goes down abruptly, as shown in FIG. 10a. 
Under the circumstance, the heating steam of the turbine gland steam evaporator 12 may lose, the amount of supply of the gland seal steam from the turbine gland steam evaporator 12 to the steam turbine 8 may fall in a short time, and this situation may damage the steam turbine 8.
The vacuum drop of a condenser 10 becomes comparatively greater by rapid reduction of the drive steam of the steam jet air ejector 14, because the ability of steam jet air ejector 14 to discharge the noncondensing gas goes down then.
In view of the foregoing, it is an object of this invention to provide a steam turbine control device and the method for a nuclear power plant.
This object can be achieved according to the present invention by providing, in one aspect, a steam turbine control device of nuclear power plant including:
a main steam system connected to lead steam generated in a nuclear reactor into a steam turbine, comprising a main steam line, a main steam isolation valve, and a main steam control valve;
a turbine by-pass system connected to by-pass the steam turbine, branched from the main steam system, and connected to a condenser, the turbine by-pass system comprising a turbine by-pass valve;
a main steam pressure detector in the main steam system;
a main steam pressure control calculating means for outputting a main steam pressure control signal dependant on the signal from the main steam pressure detector;
a reactor dome pressure detector in the nuclear reactor;
a reactor dome pressure control calculating means for outputting a reactor dome pressure control signal dependant on the signal from the reactor dome pressure detector;
a main steam isolation valve fully closed position detector for outputting a pressure control change trigger signal when the main steam isolation valve is detected to be fully closed; and
a pressure control signal calculating means for outputting a pressure control signal to control the position of the main steam control valve and/or the turbine by-pass valve comprising a pressure control signal changeover means to change over a pressure control signal from the reactor dome pressure control signal to the main steam pressure control signal when the main steam isolation valve is detected to be closed by the main steam isolation valve fully closed position detector.
In another aspect, in a nuclear power plant including a main steam isolation valve in a main steam system between a nuclear reactor and a steam turbine, there is provided a control device for controlling at least one of a first valve and a second valve, the first valve being in the main steam system and the second valve being in a steam turbine by-pass system branched from the main steam system, the control device including:
a first pressure monitor connected to the nuclear reactor;
a main steam isolation valve position monitor connected to the main steam isolation valve;
a second pressure monitor connected to the main steam system between the first valve and the main steam isolation valve;
a control means for controlling the position of the first valve or the second valve based on a first pressure signal from the first pressure monitor during normal operation of the nuclear reactor, and based on a second pressure signal from the second pressure monitor when the second pressure monitor detects that the main steam isolation valve is closed.
In another aspect, in a nuclear power plant including a nuclear reactor in a primary containment vessel, a steam turbine, and a main steam system between the nuclear reactor and the steam turbine, there is provided a method for controlling the steam turbine based on the position of a main steam isolation valve in the main steam line, the method including the steps of:
monitoring the pressure in the nuclear reactor when the main steam isolation valve is open;
closing the main steam isolation valve to isolate the primary containment vessel;
monitoring the pressure in the main steam system downstream of the main steam isolation valve when the main steam isolation valve is closed; and
controlling the amount of the supply of the steam to the steam turbine only in response to the pressure in the main steam system downstream of the main steam isolation valve when the main steam isolation valve is closed.